Analog and digital Quadrature Amplitude Modulation (QAM) methods for amplitude modulating two symbol clocks phase-locked in quadrature have been known and used since the early days of signal processing and are widely used today. For example, analog QAM is used to transfer the chroma component information in the 1953 National Television System Committee (NTSC) and the 1963 Phase Alternating Line (PAL) standard television signals and a 1977 Compatible QAM variation (C-QUAM) is still used to transfer the stereo difference information in some AM stereo radio signals. More recently, a variety of digital QAM schemes (quantized QAM) were adapted for widespread use in cellular systems and for other wireless applications, including the WiMAX and Wi-Fi 802.11 standards.
Advantageously, digital QAM may be configured with Amplitude-Shift Keying (ASK) to provide many data bits per symbol and thereby increase data transfer rates in a channel without increasing Inter-Symbol Interference (ISI). Amplitude modulating two symbol clocks in quadrature (QAM) can be equivalently viewed as both amplitude modulating and phase modulating a single symbol clock and each such modulation value (amplitude and phase) can be represented as a single point (symbol) on the phase plane diagram, as is well-known in the art. For example, by using two distinct amplitudes and four phase shift states for each of these amplitudes, a single symbol clock cycle can serve to carry one symbol having eight states; equivalent to three bits of information. In this example, a 5 MHz channel baseband can transfer data at 15 Mb/s at the expense of requiring a more robust method for reducing the impact of noise and increasing the Signal-to-Noise Ratio (SNR) to permit recovery of the significantly higher number of discrete signal amplitudes involved in each symbol clock cycle.
Proper separation of the I(t) and Q(t) quadrature components of a digital or analog QAM signal requires the coherent demodulator signal phase at the receiver to be exactly in phase with the received QAM signal carrier. Even a small demodulating phase error introduces crosstalk between the I(t) and Q(t) quadrature components recovered from a digital or analog QAM signal. Both symbol clock and carrier recovery systems in a receiver attempt to derive information about timing from the received signal, often in a similar manner. While carrier recovery is only necessary in a coherent demodulation system, symbol clock recovery is required in all schemes, and accurate clock recovery is essential for reliable data transmission. Confusion often exists between clock and carrier recovery. Clock recovery attempts to synchronize the receiver clock with the baseband symbol rate transmitter clock, whereas carrier recovery attempts to align the receiver local oscillator with the transmitted carrier frequency.
Thus, symbol clock synchronization at the receiver must be handled somehow in any QAM system. Any phase and frequency variations introduced by the channel must be removed at the receiver by properly tuning the sine and cosine components of the local QAM demodulator, which requires a local symbol clock phase reference that is typically provided by some useful version of a local Phase-Locked Loop (PLL). But this local phase reference must somehow be synchronized with the received QAM signal symbol clock. For example, early analog QAM television systems transmit a burst of the color subcarrier after each horizontal synchronization pulse for local clock phase reference synchronization.
The QAM art has evolved in various ways to increase throughput and reliability. A typical QAM data communication system includes a transmitter, a receiver, and an unknown time-invariant channel in which a complex-valued sequence of input data representing a series of symbols selected from a complex symbol alphabet (also denominated a “constellation” on the complex I-Q plane or “phase plane”) are sent through the channel to be interpreted by the receiver. Conventional QAM systems assume that channel noise is independent of input data and relatively stationary. Some distortion of the transmitted signal is typical of non-ideal channel media including wired and wireless connections.
The QAM demodulator is by far the most complex element of the QAM system. The demodulator must detect the phase and amplitude of the received signal, decode each symbol based on the phase and amplitude of the baseband symbol clock and then finally convert the symbol data back to a serial stream. The baseband symbol clock must be recovered to complete the symbol demodulation. Clock recovery is a recurring problem with any digital signal processing system.
The QAM art is replete with improvements intended to increase channel data transfer capacity while reducing receiver cost and complexity. There is an undesirable level of complexity and overhead in conventional QAM receivers for filtering signals and recovering baseband symbol clock synchronization. In applications where channel bandwidth is limited, such as pipe inspection system channels with a handful of hard-wired conductors, additional problems include correcting for a variable-length copper channel and limiting camera-end hardware complexity to facilitate the small package size necessary for movement inside pipes.
Practitioners in the art have proposed a wide variety of methods simplifying the QAM carrier and clock recovery problem. For example, in U.S. Publ. Appl. No. 2009/0,147,839 A1, Grenabo discloses an improved phase error detector for a QAM receiver but neither considers nor suggests any symbol constellation adjustments. Similarly, in U.S. Pat. No. 7,283,599 B1, Herbig discloses an improved phase error detector for a QAM receiver suitable for improving phase locking characteristics but neither considers nor suggests using an asymmetric symbol constellation. And, in U.S. Pat. No. 4,987,375, Wu et al. disclose a carrier lock detector for a QAM system employing symbol detection ratios and useful for improved reliability at low SNR but neither consider nor suggest any symbol constellation adjustments.
Practitioners in the art have also proposed a wide variety of methods for improving QAM system performance through manipulation of the symbol constellations. For example, in U.S. Publ. Appl. No. 2008/0,317,168 A1, Yang et al. disclose an integer spreading rotation technique for shaping symmetric QAM symbol constellations to enhance signal space diversity but neither consider nor suggest techniques for improving baseband symbol clock recovery at the receiver. These practitioners appear to firmly believe that the QAM symbol constellation must be as symmetric as possible about the phase plane origin to minimize the system Bit-Error Rate (BER).
Some practitioners have found certain slight asymmetries in the QAM symbol constellation to have some utility but have neither taught nor suggested using changes to the symbol constellation to improve baseband symbol clock recovery in QAM system receivers. For example, O'Hara et al. (“Orthogonal-Coded Selective Mapping (OCSM) For OFDM Peak-To-Average Power Reduction Without Side Information,” Proceeding of the SDR 04 Technical Conference and Product Exposition. 2004) propose a selective mapping (SM) method for reducing peak-to-average power (PAP) in Orthogonal Frequency Division Multiplexing (OFDM) systems that is achieved by introducing a very small asymmetry to the QAM subcarrier constellations before scrambling. But O'Hara et al. take pains to point out that this does not mean that the QAM subcarrier constellations are no longer zero-mean over time because the subsequent antipodal scrambling process returns the subcarrier symbol constellations to zero-mean symmetry again before transmission.
Other practitioners have suggested using a pilot tone in a QAM channel to improve channel estimation. For example, Tariq et al. (“Efficient Implementation Of Pilot-Aided 32 QAM For Fixed Wireless And Mobile ISDN Applications,” Vehicle Tech. Conf. Proc., 2000, VTC 2000-Spring Tokyo. 2000 IEEE 51.sup.st, Vol. 1, pp. 680-684) discloses an improved QAM system where a gap is created in the center of the information bearing signal spectrum and a pilot tone inserted therein before transmission. Tariq et al. neither teach nor suggest that their pilot tone has any relationship to the QAM baseband symbol clock; in fact, they teach using the pilot tone at the receiver only for the purpose of channel estimation and compensation. In U.S. Pat. No. 3,813,598, Stuart discloses a pilot-tone aided QAM carrier recovery system that adds a pilot tone to the QAM transmission either above or below the QAM modulator output spectrum, which may be recovered and used to deduce channel distortion effects at the receiver, but Stuart neither considers nor suggests any manipulation of the symmetric QAM symbol constellation for baseband symbol clock recovery.
In U.S. Pat. No. 6,493,490 B1, Lin et al. disclose an improved phase detector for carrier recovery in a dual-mode QAM/VSB (Vestigial Sideband) receiver system. Lin et al. discuss creating a pilot-tone aided Offset-QAM signal by first delaying the Q component by one half of a symbol, thereby offsetting the Q rail, in time, from information on the I rail, but neither consider nor suggest using an asymmetric QAM symbol constellation. Hyun et al., (“Interleaved 5820 Code For Insertion Of Carrier And Clock Pilots In 64-QAM Systems,” IEEE Electronics Letters, Vol. 27, No. 18, pp. 1635-6, 29 Aug. 1991) disclose a method for selecting symbols from a symmetric diamond-shaped symbol constellation to introduce a spectral null at the Nyquist frequency, thereby permitting the detection of a low-power clock pilot signal inserted at the null frequency, but neither consider nor suggest using an asymmetric QAM symbol constellation.